It is customary in the xerographic art to form an electrostatic latent image on a photoreceptor drum or plate comprising a charge conductive backing such as, for example, a metallic or metal-coated base having an inorganic photoconductive insulating layer applied thereto in good charge blocking contact. Suitable plates or drums can comprise, for example, an aluminum surface having a thin layer of vitreous selenium and an aluminum oxide and/or polymeric interlayer. Such elements are characterized by being capable of accepting a suitable electrostatic charge and of quickly and selectively dissipating a substantial part of the charge where light is exposed. In general, photoreceptors containing such elements are sensitive to light in the blue-green spectral range.
While selenium containing photoconductive elements are usefully employed in commercial xerography, there has been room for substantial improvement in photoconductive properties such as the range of spectral response, heat and charge stability, etc. These can be improved, for instance, by the addition of various photoconductive alloys, alloying elements or other types of additives (ref. U.S. Pat. Nos. 2,803,542 and 2,822,300). In particular, the addition of various amounts of arsenic can result in a broader range of spectral sensitivity and improve overall photographic speed and stability. Suitable alloys or homogeneous mixtures of elemental selenium with other metals suitable for this purpose can be incorporated into the usual photoconductive material and applied by conventional vacuum evaporation techniques. For example, additional inorganic coating materials can be placed in open or shuttered crucibles during an initial coating step. The xerographic substrate upon which the photoconductive material is to be deposited is conveniently placed above or in some other convenient location with respect to the potential coating vapor source. After the container has been evacuated to a suitable pressure (about 5 .times. 10.sup.-.sup.5 Torr), the vessel containing photoconductive material and/or additive is then generally heated by electric resistance to promote vaporization of the material. At least some of the vaporized material then condenses on the relatively cool substrates; such a deposition process normally requires a period of about 15-60 minutes, depending upon the amount of substrate surface to be coated and the desired thickness of coating material.
From time to time it is also found desirable to apply profile concentrations of one or more photoconductive components or separate layers of different photoconductive materials to obtain a particular desired spectrum of characteristics. In such case, the respective photoconductive materials or alloys are most conveniently applied to substrates or bases by coevaporation techniques, in which predetermined amounts of the respective photoconductive materials or alloys are placed in separate crucibles or in subdivided crucibles and exposed or heated in a predetermined sequence under vacuum. One possible modification for this purpose involves coating substrates in the presence of one or a plurality of elongated shuttered or unshuttered crucibles heated by electrical heating elements or by other conventional means, the crucibles being subdivided into a plurality of compartments or bins, each capable of carrying different pre-measured amounts and kinds of coating materials depending upon the desired final concentration. Another possible modification involves the formation of one or more trains of smaller crucibles temporarily connected to each other and containing various photoconductive materials. Both arrangements are found to be very useful in coating a plurality of substrates simultaneously with a plurality of components.
The modifications above described are very useful, however, there are serious economic and technical limitations inherent in their use. For instance, it is very difficult to maintain and efficiently operate mechanical devices such as crucible shutters for batch coating purposes due to sticking caused by the random condensation of photoconductive material within the vacuum coater, the poisonous nature of the material and the environment which must be maintained during coating. The alternative of using weighed amounts of each desired component in a plurality of open, self-heating crucibles offers a partial solution to the problem except for the substantial expenditure of time and money required to fill a plurality of crucibles with different amounts of different photoconductive materials or components thereof during each batch coating operation. In addition, it is difficult to avoid contamination and to control spattering in a timed evaporation sequence due to uneven heat distribution or hot spots of a generally unpredictable nature within individual crucibles and their contents.
The mechanical problems noted above can be avoided substantially by the use of one or more open crucibles and a timed heating sequence, preferably with irradiation heating devices such as infra red heat sources; but this is only a partial solution to the overall problem. A number of inorganic photoconductive materials, including selenium and many useful alloys of selenium are transparent or at least partially transparent to light of the longer wavelengths such as infra red. As a result, the crucible walls and bottom plus various hot spots within each crucible charge will heat up much faster than the top. This not only results in the inefficient use of energy input due to secondary radiation from the crucible walls and bottom, but may actually result in small explosions due to the buildup of gases and cause serious spattering of the coating material with resulting defects on the surfaces being vacuum coated.
It is an object of the present invention to more efficiently control vacuum coating of vaporizable inorganic materials or components thereof onto prepared surfaces or bases.
It is a further object to more efficiently utilize the radiant energy input in batch coating involving vacuum deposition of one or more vaporizable inorganic photoconductive materials.
It is a still further object to substantially limit or avoid spattering while vacuum depositing one or more selenium or tellurium containing photoconductive materials while utilizing longwave radiation substantially in the infra red region of the spectrum.